The present invention relates broadly to surface acoustic wave filters and in particular to a stagger-tuned broad bandwidth surface acoustic wave filter apparatus.
While it is well known that surface acoustic wave (SAW) transducers may be used as filters, no prior art SAW filter has simultaneously achieved a bandwidth greater than 35 percent, a shape factor less than 1.6, and sidelobes less than 40 dB on Y,Z cut lithium niobate substrate. Other SAW filter approaches may provide better performance on one of these parameters, but none can simultaneously achieve all three parameters.
It is well known that the use of apodization with a weighting function and the three transducer approach are common techniques of obtaining good shape factor, but these techniques are only useful for less than 10 percent bandwidth. Good shape factors are obtainable from known broadband techniques. The first technique is the Reeder type matching network for a broadband unapodized transducer. This approach uses a quarter wave inverter (or lumped element equivalent) in addition to the standard series inductor. However, the bandwidth and shape factor never even approached the design goals of 48 percent bandwidth and shape factor of 1.24. A linear chirp with 400 single electrode fingers in each transducer is a possible approach to meet the desired design criteria. With this device, the resulting ripple and droop on the high frequency end may be reduced with apodization, but the shape factor is not adequate--especially on the high frequency side. The case of 200 electrodes per transducer was also considered. The frequency response does not show the droop on the high frequency end, but, as maybe expected, the shape factor was worse. Since apodization and more than 400 electrodes would be needed to attain the shape factor goal, this approach is not practical due to potential difficulties in fabrication.
A phase reversal transducer (PRT) is another approach to the problem. However, in this design as the bandwidth increases, the sidelobes also increase. In a typical example, the bandwidth may be increased to 37 percent to give a shape factor of 1.57 and sidelobes of 12 dB. Although increasing the bandwidth will further improve the shape factor but the sidelobes will degrade still further. Therefore, this approach is not particularly useful.
It is also well known that a single apodized transducer will not meet the design goals for bandwidth and shape factor simultaneously. The next attempt then was to try stagger tuning of two different broad band apodized transducers connected in parallel. While the bandwidth was realized and sidelobes were greatly reduced, however, the shape factor was similar to the linear chirp filter and did not meet the design goal.